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Full text of "Practical Organic Chemistry"

DETERMINATION OF MOLECULAR WEIGHT

tripod.    In the figure the lower part of the porcelain jacket and
the asbestos tray are made transparent to show the position of
the burners and the concentric rings of asbestos below the tray.
The asbestos  has a circular hole in the centre, which admits
the  lower end of the boiling-tube.     Two  asbestos  chimneys                   t | IL
are fixed upright at the diagonal corners of the tray to carry
off heated air and two burners are placed below the other two                     /i'l
corners.    The boiling-point of the solvent is first ascertained.                     |/|
For this  purpose  benzene  may be  used.        The  Beckmann                     fif
thermometer must be adjusted so that, when in the boiling liquid,                     !11
the thread occupies the lower half of the scale.    In order to                    ;/4|
adjust it, the bulb must be placed in water warmed gradually                     '^|
67 above the boiling-point of benzene, and the bead  then                    'tf '.
detached as already explained in the description of the freezing-                      \f
point method.                                                                                                 > ^
The boiling-tube is  carefully dried and  weighed with  the                     {4?
beads.     Sufficient benzene is poured in to cover the bulb of the         .             ;',<
thermometer, which is pushed down a little way into the beads.                      ^*
The condenser is attached to the side limb.    A layer of i2 cm.                       fl
of benzene is poured into the outer jacket, and the condenser                      , $
fixed in position. The same water supply may be made to
traverse both condensers. The two burners under the tray are
lighted and the temperature regulated so that the benzene in the
outer jacket boils briskly, whilst at the same time sufficient heat
finds its way to the boiling-tube, through the gauze ring outside
the concentric screens of asbestos below the tray, to keep the
benzene in the state of steady ebullition. In about J hour from
the time the benzene boils in the inner tube the first reading may
be made, and a fresh reading every five minutes until the
temperature is constant, z>., does not vary more than 0*01. As
the atmospheric pressure may produce considerable variations in
the reading, it is important to observe the barometer occasionally
during the experiment, and to make a correction, which is about
0*043 fr every i mm. below 760.
The temperature being constant, a pellet (0*10*2 gram) of
fused naphthalene is carefully weighed and dropped into the boil-
ing-tube through the condenser without interrupting the boiling.
These pellets are conveniently made in a small bullet-mould.
The boiling-point will rise and after a few minutes will remain
stationary. The temperature is noted. A second and third